Kilauea Volcano

The current eruption along Kilauea's East Rift Zone
continues with passive effusion of lava. The lava is transported
underground to the coast along the western edge of the flow
field, where it enters the ocean after cascading over an ancient
sea cliff. The National Park Service has had to close off access
to the area because of noxious fumes produced where lava is
burning the road.

Kilauea is world-famous as the "drive-in" volcano because of
the passive character of its eruptions and the ease and safety of
viewing the volcanic activity. Much to the surprise of those who
have seen its approachable eruptions, Kilauea also has a history
of violent phreatic (steam) or phreatomagmatic (steam and
magma) explosive eruptions. May 10, 1995, was the 71st
anniversary of the most recent of these violent eruptions. On
that day, a violent phreatic eruption began in Halema'uma'u
that sent repeated columns of ash high into the sky. The
explosions continued for 18 days, with the largest occurring on
May 18. The steam explosions hurled rocks up to eight tons as
far as 0.6 miles from the crater; these blocks still surround
Halema'uma'u.

By far the most devastating historical eruption occurred at
Kilauea's summit in 1790. Its enormous ash columns were
somewhat similar to those of the eruptions at Mount St. Helens
in 1980. The 1790 eruption began with phreatomagmatic
(mixtures of water and magma) explosions of fine, hot particles
of volcanic glass and ended with phreatic explosions that
ejected small to large fragments and blocks of pre-existing
rocks. The surface of the volcano was scoured by hurricane-like
blasts of hot gases and ash, called base surge, that formed from
collapsing columns of ash. The 1790 ash columns are estimated
to have been about 30,000 feet high, based on reports of
visibility from Kawaihae.

There is evidence of numerous explosive eruptions at
Kilauea Volcano in the more distant past. Another prehistoric
explosive eruption deposited a thin layer of ash about 1,200
years ago, whereas a much more extensive deposit, the
Uwekahuna Ash, erupted between 2,100 and 2,700 years ago.
The eruptions which produced the Uwekahuna Ash were much
larger than those of 1790, but most of the deposits are buried
by younger lava flows of Kilauea and Mauna Loa.

The Pahala Ash formed from even larger explosive
eruptions earlier in Kilauea's history. Two one-to-two-meter
thick ashes underlie the Uwekahuna Ash, but overlie the
thickest prehistoric ash, a reddish sequence of ash up to 85 feet
thick where it is exposed in the Hilina Pali. The Pahala Ash is
thus a composite of these separate ashes. The youngest is dated
between about 3,500 and 4,800, and the oldest thick section is
about 39,000 years old. The Pahala ash occurs in a scientific
drillhole located near the Hilo airport, where it is still about
four feet thick. The 39,000-year age on this thick unit is derived
from the drill core.

Beneath the Pahala Ash is a sequence of even older ashes,
including the Mo'o Ash, the Pohaka'a Ashes (seven layers), the
Kahele Ash, and the Halape Ash. These ashes are older than
39,000 years old, although how much older is not known. All
are exposed in the Hilina Pali.

The term "Pahala Ash" has been widely used to describe
nearly all the thick soils on Hawaii. The Pahala ashes on the
south flank of Mauna Loa and those on Kilauea are all from
explosive Kilauea eruptions. On the other hand, ashes near
Hilo and along the Hamakua coast are from explosive eruptions
of Mauna Kea. The ash layers in Hilo, commonly called the
Homelani Ash, consist of many layers from different explosive
eruptions. The highly variable thickness of the ash is caused by
having different ash units represented. For example, the
thickest ash, commonly up to 20 feet thick, consists of ashes
from Mauna Kea that are older than about 120,000 years. This
layer underlies a section of the 39,000- year old Pahala Ash
from Kilauea Volcano, which lies, in turn, under several ashes
from Mauna Kea that are roughly 20,000 and 10,000 years old.
Depending on the age of the youngest underlying lava flow, all,
or some, of these units may be present.

In the past, the entire Homelani Ash was correlated with
the Pahala Ash on Kilauea. We now know that the thickest
section of the Homelani Ash is from Mauna Kea, lies beneath
the Pahala Ash, and is between 120,000 and 200,000 years old.
It is likely that this thick ash unit erupted between 135,000 and
165,000 years ago, when the summit of Mauna Kea was covered
by glaciers. Melt water from subglacial eruptions triggered
explosive phreatomagmatic activity.

The preceding article was published in the June 1995
issue of Lithosphere, the official bulletin of the
Fallbrook [California] Gem and Mineral Society, Inc; Richard Busch
(Editor).